The present invention relates to a circuit for controlling an automatic iris of a video camera, and more particularly, such a circuit with provisions for inhibiting dark current from effecting the iris control.
Surveillance, industrial and portable cameras, are used in an extremely wide range of environmental conditions in the normal course of operation. For example, a surveillance or portable camera may be subjected to high levels of illumination in the daytime and subjected to extremely low levels of illumination at night. The imagers used for such cameras include imaging devices such as pickup tubes (vidicons) and solid-state sensors such as charge coupled devices (CCD) and metal-oxide semiconductor devices (MOS). Such imagers have a relatively limited dynamic range as compared to the variation of illumination levels under which the camera is expected to operate. Consequently, to make maximum use of the imagers dynamic range, cameras typically include an iris comprising a mechanical diaphragm coupled in the optical path of the imager for controlling the amount of light passed through to the imager so that the imager operates within a preferred illumination range. The iris can be automatically controlled in a degenerative feedback loop manner in response to the level of the video signal supplied from the imager. One such automatic iris controller is described in U.S. Pat. No. 2,885,471 issued on May 5, 1959 in the name of G. King. The King arrangement generates an iris control signal in response to the peak-to-peak value of the video signal. Thus, when the imager views a scene containing highlights, i.e., high brightness portions of a scene, the iris controller senses the highlights and closes the iris to a level commensurate with the dynamic range of the imager, thereby providing adequate contrast in the region of the highlights.
In camera markets having a relatively high volume, such as the surveillance, industrial and consumer markets, it is desirable that a camera be adaptable for use with standard optical systems. Among the optical systems currently enjoying widespread use are automatic iris lenses such as the type manufacturered by Vicon Industries, located in Melville, New York and the Cosmicar Lens Division of Asahi Precision, Tokyo, Japan. As illustrated by element 20 in FIG. 2, the automatic iris control circuitry typically includes an input AC coupled to receive the video signal, a clamp for esablishing a given potential for the most negative portion of the video signal (typically the blanking portion) and a peak detector for developing a DC voltage representative of the video signal peaks (highlights). This DC signal is applied to control the iris opening.
As shown in FIG. 1, video signal 10 includes periodic blanking portions 12 corresponding to the horizontal retrace intervals of the television signal, interspersed with lines of image representative signal levels corresponding to active portions 14 of the television signal.
In addition to supplying the video signal, the pickup tube or solid-state imager supplies another component commonly referred to as "dark current". Dark current is a signal which is supplied by the imager even in the absence of light, mainly due to thermally generated electron-hole pairs in the imaging photosensitive surface. Typically, dark current comprises a DC signal level which causes the active video portion 14 of signal 10 to "ride on top of" a DC component (V.sub.DC) dependent upon the level of dark current. When dark current component is relatively small, the automatic iris control circuitry is able to provide an iris control signal which can follow substantially the full dynamic range of amplitude variation of the active portion 14 of the video signal. However, the level of dark current accompanying a video signal increases with increases of temperature and when the camera is operated under environmental conditions having temperatures above room temperature, the dark current component can become a substantial portion of the video signal, such as indicated by waveform 16. Thus, the DC level of the active portion 14 of video signal 16 is significantly raised, yet the level of the blanking interval is relatively unaffected. This is so in pickup tubes because, during the blanking interval the cathode of the tube is biased to cut-off. Thus, there is no dark current output during this time. In solid-state imagers, the blanking interval level is also not dark current dependent because charges are not read-out during the blanking interval, either due to stopping of the read-out clock or due to the fact that all the charge samples for a given line have already been read-out.
Automatic iris control circuitry of the type described by King, clamps the most negative portion of signal 16, i.e. blanking portion 12, to a given value and peak detects the maximum level of active portion 14 of signal 16 for providing the DC iris control signal. Since almost half of the peak value of signal 16 does not correspond to image brightness variations, approximately 50% of the dynamic range of iris control can be lost to dark current under high temperature environmental conditions.
Therefore, it is desirable to eliminate the adverse reduction in iris control dynamic range caused by the dark current component of the video signal.